Gastrointestinal track ulcers, gastritis, and reflux esophagitis occur while the balance between aggressive factors (e.g., gastric acid, Helicobacter pylori pepsin, stress, alcohol and tobacco) and protective factors (e.g., gastric mucosa, bicarbonate, prostaglandins, the degree of blood supply, etc.) is destroyed. Therefore, a therapeutic agent for gastrointestinal damage such as gastrointestinal track ulcer, gastritis and reflux esophagitis is divided into a drug for inhibiting the aggressive factors and a drug for enhancing the protective factors.
Meanwhile, it is reported that gastrointestinal track ulcers, gastritis and reflux esophagitis occur ulcers even without an increase in secretion of gastric acid. Thus, as much as the aggressive factor increases, a reduction in protective factors due to a pathological change of the gastric mucosa is thought to play an important role in the occurrence of gastric ulcers. Therefore, in addition to drugs for inhibiting the aggressive factor, drugs for enhancing the protective factors are used for the treatment of gastrointestinal ulcer and gastritis. As the drugs for enhancing protective factors, mucosal protective drugs which are attached to the ulcer site to form a physicochemical membrane, drugs that promote the synthesis and secretion of mucus have been known.
On the other hand, Helicobacter pylori (H. pylon), which is a bacteria present in the stomach, has been known to cause chronic gastritis, gastric ulcer, duodenal ulcer and the like, and a number of patients with gastrointestinal damages are infected with H. pylori. Therefore, these patients should take antibiotics such as clarithromycin, amoxicillin, metronidazole and tetracycline, together with anti-ulcer agents such as a proton pump inhibitor, or a gastric pump antagonist. Consequently, various side effects have been reported.
Therefore, there is a need to develop anti-ulcer drugs which inhibit the secretion of gastric acid (e.g., proton pump inhibitory activity) and enhance protective factors (e.g., an increase in mucus secretion) and at the same time have disinfectant activity against H. pylori. 
In this connection, Korean Patent No. 10-1613245 discloses that a 4-methoxypyrrole derivative or a pharmaceutically acceptable salt thereof has excellent anti-ulcer activity (i.e., proton pump inhibitory activity, etc.) and disinfectant activity against H. pylori, and thus can be effectively used for the prevention and treatment of gastrointestinal damage due to gastrointestinal track ulcer, gastritis, reflux esophagitis or Helicobacter pylori. 
In the preparation of the 4-methoxypyrrole derivative described in the above patent, the following compound is prepared as an intermediate.

According to the description of the above patent, the intermediate is prepared from 2,4-difluorophenylglycine, and the preparation method consists of four steps in total (Steps (8-1) to (8-3) of Example 8 described in Korean Patent No. 10-1613245). However, according to the preparation method of the above patent, the total yield is as low as 9.0%, a high-temperature reaction is required as a whole, and thus expensive equipment is required. Especially, (trimethylsilyl)diazomethane is used as a reactant, but this reagent is not only expensive but also explosive and thus is not suitable for industrial mass production.
Given the above circumstances, the present inventors have conducted intensive studies on a new preparation method capable of preparing the above intermediate. As a result, the inventors have found a preparation method in which a high-temperature reaction is not required as a whole as in the preparation method described later, and inexpensive, non-explosive reagent is used instead of (trimethylsilyl)diazomethane, and further, the yield is improved as a whole, thereby completing the present invention.